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  1. #14
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Humor me and let me blab my opinions as an amateur. I am a homeowner too and must admit I would be timid about going as low as that. Would be very interested in paying a pro to repeat the load calc, and especially do the Manual S calculation to ensure the chosen equipment has enough capacity after considering latent and sensible load separately, equipment de-rating due to temperature conditions, etc.

    And I would be very tempted to design the duct system for something extra-large, then 1) you could potentially use higher CFM/ton for sensible efficiency, and 2) an upgrade may be easier should you make an addition, or change your lifestyle, or want to sell to someone who thinks "old school". Keeping the AC at 4, 5 tons or whatever you end up choosing. The reason is that upsizing an AC is fairly common (tho sometimes misguided), while many duct systems are undersized, and I don't know of any substantial downside.

    One last way to hedge my bets might be to choose a 2-stage AC which is much less sensitive to oversizing problems. In a weird way having 2 AC systems (most houses here do) contains some of those advantages.

    Personally I have 6.5 tons for 3300 sqft in S.Texas, while my own Manual J says 4.5-5.0 is enough (your house will probably be better constructed). I *know* one of my 2 systems is well oversized, by measuring runtimes I estimate I can maintain comfort conditions with 5.5-6.0 tons. Cannot say exactly why the difference, but I have as much confidence in my own observation as I do in a mathematical model, even if that model is the venerable Manual J. In my neighborhood anyway they usually would install around 8 tons, for what that matters.

    Best of luck -- Pstu

  2. #15
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,122
    Originally posted by pstu

    Personally I have 6.5 tons for 3300 sqft in S.Texas, while my own Manual J says 4.5-5.0 is enough
    Yep I have posted this here before many times you can't go strictly by manual J in Tx. well in Houston anyway or you will be in trouble you better be using a little experience and common sense or your butt will be in trouble and this is a fact.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  3. #16
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    What is your actual sensable heat gain?

    Are basing the need for a "4 ton" system off the actual sensable load that HVAC Calc came up with?

    In HVAC Calc, when you look at the report, it will have something like (4 tons) at the bottom of it under the total heat gain for the house.
    You need to totally ignore that, it is most likely not the size system you need as it just adds the sensable and latent loads together and divides it by 12,000.
    To properly size a system, you need to match up equipment that will satisfy the sensable heat gain and still have enough latent capacity to satisfy the latent load. In Houston, you would want lots of "extra" latent capacity.

    You need to sit down with your HVAC contractor and go over the loads with them and make sure you get a system that is matched up properly for your needs.
    If more government is the answer, then it's a really stupid question.

  4. #17
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,429
    Originally posted by downtown
    I have learned so much that I am actually EXCITED to spend $30K on an heating and AC system!

    that I just need a 4 ton system for cooling.

    So I am I wrong to assume that a 4 ton system will work?

    Here are additional details:
    rancher w/ basement
    r4 insulation on outside, r13 in walls
    west side partially shaded
    aluminium radiant barrier in ceiling under rafters (over 5,000 sqft of the stuff!!)
    latest andersen windows, only two facing north, most facing west.
    You must be > 1400 miles NOrth of Minneapolis
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #18
    Join Date
    Apr 2002
    Location
    Pottsboro TX
    Posts
    181

    HEAT/COOL CALCs

    System installation, in my opinion, will ALWAYS come up short delivering the stated capacities. Lack of attention to details, poor system design installation, some sloppy work on ducts, charging, etc. will reduce effective capacity. All of those items are not accounted for by Calc.
    The house itself will fall short of the stated R-values too. You'll always find places where the insulation does not fit tight, windows not installed as per manufacturers instructions, penetrations are not sealed against infiltration, windows delivered without low-E coating, windows installed inside-out, etc.
    Be careful undersizing. I watched the construction of my house daily and still found details I had to have changed that otherwise would have lowered overall R-values.
    Did you have a HVAC engineer design the HVAC system? On a house of that Sq. Ft. it could save you a few headaches.
    Phil

  6. #19
    Two stage 5 ton ????

  7. #20
    Join Date
    Jul 2006
    Posts
    71
    I am waiting for a HVAC contractor to provide manual j cals or at least review/critque my analysis. I agree my house my not follow the calcs exactly and it may be safer to slightly overdesign the system. I just get disturbed when contractors tell me I am nuts without perform or reviewing my own analysis. I had one contractor admit that he fudges his numbers to purposely over design the system in order to pass inspection. In my neck of the woods, manual js are required by the county and the inspector will match the calcs with the installed system.

  8. #21
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,784
    In my area, because of our design temps, we don't really need to use manual S.

    But with your design temps you NEED to use it.

    Just out of curiousity, I looked up a York Affinity 13 SEER, and at your posted design, a 4 ton, would only have a capacity of 38,900 BTU's total.

    And a 5 ton would be 45,000 BTU's.

    So you may need a 5 ton unit, to get the 4 tons of cooling you need to reach a 73° indoor temp when its 100° outside.


    So what was the sensible and latent btu's hvac calc said you need ?
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  9. #22
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Good point beenthere, I didn't notice his 73º indoor design temp.
    While I would never ever advocate such a low indoor design temp, if someone does insist on it, you MUST look at the performance of the equipment at those indoor and outdoor conditions. Using the ARI rated capacities will result in a system that will not do what you are wanting.
    Many manufacturers don't even make the performance data available for thier equipment at conditions other than ARI conditions.
    This is sounding more and more like a good application for a 5 ton outdoor unit with a smaller indoor unit.
    A Trane 5 ton XR13 heat pump with a TWE049E air handler running 1800 CFM gives 51500 btuh total and 43,400 btuh sensable capacity at his design conditions. At those design conditions, thats a LOT of extra latent capacity for humidity control, wich is very desireable in humid areas.

    [Edited by mark beiser on 08-05-2006 at 09:07 PM]
    If more government is the answer, then it's a really stupid question.

  10. #23
    Join Date
    Aug 2006
    Location
    San Diego,CA
    Posts
    9
    Maybe I missed it but is your house single story, two story, multi-level or ? And what about ceilings ? are they cathederal,vaulted or otherwise? And what about attic space ? is there any ? Maybe the expense and or labor & material to duct a house of this size is comperative to a "split-system" or maybe the restrictive loss of static pressure due to the irregularities of modern home construction i.e; squeezing down ,shrinking or otherwise mangling the duct runs to get them where they need to be. Like I've seen in multi family housing tracts.

  11. #24
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    so, you are designing for the hottest day?
    or, are you willing to let indoor temp drift when temp 95+ ?

    my 1984 NOAA manual shows 88F to be the normal temp for Jul in Richmond, south of you -- with 105F being the 30y hottest.



    [Edited by cem-bsee on 08-05-2006 at 07:13 PM]
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  12. #25
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Heh, for some reason I thought we were talking about a house in Houston, I just noticed that it is for Fredricksberg, VA.

    Why on earth would you use 100º and 73º indoor as your design conditions?

    If you design the system for the very worst case conditions you are ever likely to see, then your system is oversized all of the rest of the time. The worst thing you can do in a humid area is grossly oversize the HVAC system.

    The ACCA design conditions for your area are 93º outdoor temp, 75º indoor temp with 38 grains difference at 50% indoor RH.
    If more government is the answer, then it's a really stupid question.

  13. #26
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,429

    Thumbs up Review

    Originally posted by downtown
    I am waiting for a HVAC contractor to provide manual j cals or at least review/critque my analysis.

    I agree my house my not follow the calcs exactly and it may be safer to slightly overdesign the system.

    In my neck of the woods, manual js are required by the county and the inspector will match the calcs with the installed system.
    What are the BTUh calculated for
    1. Roof _______ based on R- __
    2. Walls ______ based on R- __
    3. Windows ____ based on SHGC __
    4. Infiltration _____
    5. Ducts ___ based on R-___
    6. Internal Loads _____ based on ____ of people and
    ___ for appliances
    ...
    Amount of glass area
    7. North ___ sq.ft.
    8. South ___
    9. East ____
    10. West ____ ?

    10-minute check will follow, if you reply/write me.

    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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